[SI-LIST] Re: RF via

Alex,

In your original post I thought you were looking for
RF grounding. This sounds like you are talking about
a signal via. If so:

1. You need a path for the return current close to the
    signal via. If you have room, you can place ground
    vias around the signal via to form a vertical transmission
    line in the board. Controlling this impedance gets more
    critical as the board gets thicker and the frequency goes
    up.
2. The size and distribution of pads and anti-pads can be 
    designed.
3.  In general, transitions from microstrip into the via tend
     to be inductive. Leave the pad in the groundplane layer
    in place and minimize the anti-pad dia. You actually want
     extra capacitance to compensate the inductance.
4. In general, stripline transitions tend to have excess capacitance.
    So you remove the pad or pads in the adjacent ground plane
    layers and increase the anti-pad diameter.
5.  I do this on a field-solver by optimizing for return loss across
     the frequency range of interest.

I have used this general approach for transitions up to 10 GHz and
up to 16 layers.

Dan

Dan Swanson                     EMAIL:   d.swanson@xxxxxxxx
Bartley RF Systems, Inc.        TEL:   978-834-4085     
37 South Hunt Road              FAX:   978-388-7077
Amesbury, MA  01913




On Monday, September 24, 2001 6:23 PM, Alex Horvath
[SMTP:alexh@xxxxxxxxxxxxxxxx] wrote:
> I found a source that describes an RF via (which is bad terminology, I
know)
> as follows:
> 
> 1. Large anti-vias (do they really mean anti-pads).
> 2. No pads on power and ground planes.
> 3. No pads on signal planes that are not used.
> 
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Perry Qu
> Sent: Thursday, September 20, 2001 9:54 AM
> To: DSWANSON@xxxxxxxxxxxxx
> Cc: 'alexh@xxxxxxxxxxxxxxxx'; Si-List
> Subject: [SI-LIST] Re: RF via
> Importance: High
> 
> 
> 
> Hi!
> 
> I would imagine that RF via is similiar to a coax structure, where you
have
> several return vias surrouding the signal vias. This is to match 50 ohm
> impedance as close as possible and reduce the discontinuity effect of the
> vias.
> 
> Regards
> 
> Perry
> 
> Dan Swanson wrote:
> 
> > Physically short, close to the component.
> > Lowest impedance path possible to true ground,
> > not the local ground. Via always has some
> > finite inductance.
> >
> > Dan
> >
> > On Wednesday, September 19, 2001 5:14 PM, Alex Horvath
> > [SMTP:alexh@xxxxxxxxxxxxxxxx] wrote:
> > >
> > > What are the particulars of a "rf via"?
> > >
> > > I'm using a component which switches at 2 Ghz that recommends the use
of
> > > this type of via.
> > >
> > >
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> --
> Perry Qu
> 
> Product Integrity         |      600 March Road
> Alcatel Canada            |      Ottawa, ON K2K 2E6, Canada
> 
> DID: (613) 7846720        |      FAX: (613) 5993642
> 
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